The present invention relates to helical screw type compressors. More specifically, the present invention relates to a multi-screw compressor having, e.g., a male rotor and at least two female rotors.
Helical type compressors are well known in the art. One such helical compressor employs one male rotor axially aligned with and in communication with one female rotor. The pitch diameter of the female rotor is greater than the pitch diameter of the male rotor. Typically, the male rotor is the drive rotor, however compressors have been built with the female rotor being the drive rotor. The combination of one male rotor and one female rotor in a compressor is commonly referred to as a twin screw or rotor, such is well know in the art and has been in commercial use for decades. An example of one such twin rotor commonly employed with compressors in the HVAC (heating, ventilation and air conditioning) industry is shown in FIG. 1 herein, labeled prior art. Referring to FIG. 1 herein, a cross sectional view of a male rotor 10 which drives an axially aligned female rotor 12 is shown. Male rotor 10 is driven by a motor, not shown, as is well known. Male rotor 10 has four lobes 14-17 with a 300.degree. wrap and female rotor 12 has six flutes 18-23 with a 200.degree. wrap. Accordingly, the compression-discharge phase of the axial sweep with respect to male rotor 10 occupies 300.degree. of rotation, with the timing between the closed discharge port and the closed suction port occupying the remaining 60.degree. of rotation. The resulting gap between the male and female rotors requires oil to be introduced into the compression area for sealing, however, the oil also provides cooling and lubricating, as is well know. However, the introduction of this oil requires the use of an oil separation device, to separate the oil from the refrigerant being compressed in HVAC compressors. The primary benefit of the twin rotor configuration is the low interface velocity between the male and female rotors during operation. However, the twin rotor configuration is not balanced and therefore incurs large radial bearing loads and thrust loads. The obvious solution to alleviating the beating load problem would be to install sufficiently sized beatings. This is not a feasible solution, since the relative diameters of the rotors in practice result in the rotors being too close together to allow installation of sufficiently sized bearings.
The prior art has addressed this problem, with the introduction of compressors employing `so-called` single screw technology. Referring to FIGS. 2 and 3 herein, labeled prior art, a drive rotor 24 with two opposing axially perpendicular gate rotors 26 and 28 is shown. Rotor 24 is driven by a motor, not shown, as is well known. Rotor 24 has six grooves 30 and each gate rotor 26, 28 has eleven teeth 32, 34, respectively, which intermesh with grooves 30. The gate rotors 26 and 28 are generally comprised of a composite material which allows positioning of the gate rotor at a small clearance from the drive rotor. This clearance is small enough that the liquid refrigerant itself provides sufficient sealing, the liquid refrigerant also provides cooling and lubrication. The rearward positioning of gate rotors 26 and 28 and the positioning on opposing sides of drive rotor 24, (1) allows equalizing suction of pressure at both ends of rotor 24 thereby virtually eliminating the thrust loads encountered with the above described twin screw system and (2) balances the radial loading on rotor 24 thereby minimizing radial bearing loads. However, the interface velocity between the gate rotors and the drive rotor are very high. Accordingly, a common problem with this system is the extensive damage suffered by the rotors when lubrication is lost, due to the high interface velocities of the rotors.